Adults with obstructive pulmonary diseases commonly exhibit a decreased ventilatory response to hypercapnia and/or hypoxia. Abnormal lung mechanics and diminished respiratory drive have both been blamed for this decrease. It is not known, however, whether children with the same diseases also show a decreased ventilatory response when challenged with hypercapnia and hypoxia. The object of this research is to measure respiratory drive, ventilatory responsiveness, and pulmonary functions of asthmatic and normal children to see how these three interrelate. Indices of respiratory drive are obtained by measuring the pressure developed during the first 100 milliseconds of inspiration (P100) while the airway (mouthpiece) is occluded and by measuring the work of breathing. P100 and work of breathing are determined with and without the addition of artificial airway resistance during quiet breathing and also when breathing is increased by exercise or through hypercapnic and hypoxic stimulation. Ventilatory responsiveness is assessed by measuring ventilation during exposure to several steady-state levels of hypercapnia and hypoxia. Pulmonary functions are measured with a constant-volume body plethysmographh and with an automated spirometer. Electrical signals from the masspectrometer, pneumotachygraph and differential pressure transducers are fed directly into a small digital computer for on-line computation and display as aveolar gas tensions, minute ventilation, P100 and work of breathing. Data from the pulmonary function tests are also computer processed.